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. Interactions with water are crucial for the properties,transformation, and climate effects of atmospheric aerosols. Here we presentthe high-humidity tandem differential hygroscopicity analyzer (HHTDMA) and a newmethod to measure the hygroscopic growth of aerosol particles with in siturestructuring to minimize the influence of particle shape. With thisapproach, growth factors can be measured with an uncertainty of 0.3 %–0.9 %over a relative humidity (RH) range of 2 %–99.6 % and with an RHmeasurement accuracy better than 0.4 %. The HHTDMA instrument can be used in hydration, dehydration, andrestructuring modes of operation. The restructuring mode allows us toinvestigate the effects of drying conditions on the initial microstructureof aerosol particles and specifies the optimal parameters that provide theirrearrangements into compact structures with a nearly spherical shape. Theseoptimal parameters were used in hygroscopic growth experiments bycombining the restructuring mode with a conventional hydration or dehydrationmode. The tandem of two modes allowed us to measure the particle growthfactors with high precision as well as to determine the thickness of thewater adsorption layer on the surface of compact crystalline particles. To verify the HHTDMA instrument we compared the measured ammonium sulfate growthfactors with those obtained from the E-AIM-based Köhler model (E-AIM: Extended Aerosol InorganicsModel). Averaged overthe range of 38 %–96 % RH, the mean relative deviations betweenmeasurements and model results is less than 0.5 %. We demonstrate this precision by presenting data for glucose, for which bulkthermodynamic coefficients are available. The HHTDMA-derived activitycoefficients of water and glucose were obtained for both dilute andsupersaturated solutions and are in good agreement with those reported in theliterature. The average deviation between the measured activity coefficientsand those obtained by the bulk method is less than 4 %. For dilute solutionin water with an activity range of 0.98–0.99, the hygroscopicity parameter ofglucose and the molal osmotic coefficient were obtained with an uncertainty of 0.4 % and 2.5 %, respectively.

atmospheric measurement techniques

High-humidity tandem differential mobility analyzer for accurate determination of aerosol hygroscopic growth, microstructure, and activity coefficients over a wide range of relative humidity